This invention relates to a method of rotational molding using a unique resin blend and to the resulting molded objects.
Rotational molding is, in theory, a very straightforward process. To make an object using rotational molding, all that is required are: (1) a mold to hold the material in the configuration desired when the processing is complete; (2) a source of heat to melt the resin or plastic under control conditions; (3) a machine to distribute the material uniformly over the surface of the mold; and (4) a method of cooling the resin under controlled conditions.
There are several advantages which may be derived using rotational molding. The tooling costs are economical and a hollow, one-piece object results which is virtually stress-free. Uniform wall thicknesses can be provided, and there is substantial design flexibility, allowing molding of hollow, one-piece objects ranging from relatively small objects to intricate designs and to large and complex shapes.
Common rotationally molded products include, as illustrative examples, shipping drums, storage tanks and receptacles, material handling bins, and housings. Consumer products made using rotational molding range from furniture to toys and to marine accessories.
However, there are a variety of problems, difficulties and/or complications involved when using rotational molding. One significant problem derives from the process itself. The resin which is the standard commercially available is provided in relatively large-sized particles, typically in the form of pellets. The current result is that resin producers typically manufacture resins with a pellet size ranging from 35 to 60 pellets/gram. As hereinafter discussed, particles of this size do not process well given the processes currently being used by rotational molders. Accordingly, such relatively large pellets are ground to provide powders ranging from 22 mesh up to about 35 or even 50 mesh. The grinding processes involved typically result in a relatively wide bell-shaped particle distribution curve. In the rotomolding cycle, the smaller ground particles melt first, much like ground ice melts faster than ice cubes.
While such finely ground particles are widely used in the rotomolding process to achieve this faster melting quality, the use of such ground particles presents several problems. The particle size distribution achieved following grinding varies widely from one grinder to another. Indeed, sometimes the distribution curve will vary significantly even when the same grinder is used, such as, for example, which can occur when the mechanical set-up conditions are altered slightly. This variation in the particle size distribution will effect the delivery system in the rotomolding process which can cause variations in the wall thickness of the object being molded. Further, the grinding process adds not only significant expense, but can lead to inventory problems simply because of the volume of resin needed to be ground.
Still further, and importantly, the fines in such ground resin can create processing and housekeeping problems, simply due to the level of dust involved. Such resulting dust can likewise create a potential safety issue.
Yet, despite the continuing need for a solution to these very real problems, no solution exists insofar as the present inventor is aware. Rather, the resin source used in rotomolding continues to be ground particles.
In general, the present invention is predicated on the discovery that a resin blend capable of achieving rotationally molded parts having impact strength and other characteristics, similar to that achieved with ground resin particles, can be obtained by eliminating much of the ground resin particles previously considered essential. More particularly, it has been found that rotationally molded parts having adequate processing and strength and other product characteristics can be obtained utilizing a blend of resin pellets in the size range obtained from the resin manufacturer while utilizing only a minor amount of ground particles.
Thus, as one example, when using a rotomold grade of polyethylene, rotomolded parts having only 20% to 30% ground particles achieve low temperature impact strength characteristics essentially the same as those achieved with a rotomolded part made entirely from ground particles. This is particularly surprising, perhaps evidencing some synergistic processing and/or other properties, in view of the dramatic drop-off in properties that result when the ground particle component is reduced to 10% or less.
The present invention provides a method of rotational molding using a resin blend which avoids the problems of prior rotational molding techniques that employ ground resin. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.